// Array-based stack implementation
template <class Elem> class AStack: public Stack<Elem> {
private:
  int maxSize;              // Maximum size of stack
  int top;                  // Index for top element
  Elem *listArray;          // Array holding stack elements

public:
  AStack(int size =DefaultListSize)   // Constructor
    { maxSize = size; top = 0; listArray = new Elem[size]; }

  ~AStack() { delete [] listArray; }  // Destructor

  void clear() { top = 0; }           // Reinitialize

  bool push(const Elem& it) {         // Put it on stack
    if (top == maxSize) return false; // Stack is full
    else { listArray[top++] = it;  return true; }
  }

  bool pop(Elem& it) {                // Pop top element
    if (top == 0) return false;
    else { it = listArray[--top];  return true; }
  }

  bool topValue(Elem& it) const {     // Return top element
    if (top == 0) return false;
    else { it = listArray[top-1];  return true; }
  }

  int length() const { return top; }  // Return length
};